Honeywell ST9400C Problems & Fixes

Today on, “Why won’t my things work?” we’re discussing thermostats and their issues. Specifically, we’re going to be talking about Honeywell st9400c problems. Whether that means you’ve got a blank display, issues with radiators or more, we’ve got you covered. Believe it or not, those of us at DreamyHome have dealt with this more than once, so we’ll help you get things sorted.

The most common Honeywell ST9400C problems are a blank LCD display, displays not matching reality (says heating is on, but it’s off), the “internal fault” message, and a “not valid” message.

Those were a lot of words that don’t really mean anything until it’s explained, so let’s get right into it.

Common Honeywell ST9400c Problems

Before we get too far into things, there’s a guide right here for owners who are experiencing technical difficulties. While most of the stuff in here is a bit vague, it’ll give you an idea of where to start. Whether that means you need to just call an installer or perform a bit of technical MacGuyvering, you’ll at least have a solid jumping-off point for troubleshooting.

Blank Display

Long story short, this is due to one of two things: your heating system has lost power, or there’s a fault in the Honeywell device. There’s not much you can do here beyond ensuring your device has power and that it’s not simply unpowered.

This particular thermostat requires a 230-volt power supply to function. Now, there are a few things to take a look at before calling in an installer or engineer. First, double-check that your breaker hasn’t flipped. This is common in older homes that weren’t set up to handle the mass of electronics common in modern houses. If it’s flipped, turn it back on and see if that solves the issue. If it continually flips, you’ve got too many things on the same circuit and will need to do a bit of “electronic musical chairs” with your devices to get things set.

You can also, in theory, test this with a multimeter. You’ll need to turn the power off at the main first, and then test the power supply for your thermometer. If it comes back with a reading, then it’s likely that there is something wrong with the power supply. If it reads as an open circuit, that’s normal and it’s likely that the issue lies in your thermostat.

This should not be done by people who aren’t familiar with electrical work. You don’t need to be a certified engineer to test a circuit, but if you don’t know what a multimeter is (or don’t have one), it may be best to call for help.

Thermostat Says Heating is On, It’s Not

This particular issue means that there is either a malfunction in your boiler or heating system, or that you’ve turned the temperature controls off or too low.

If you notice that your taps are running with cold water, this is another sign that there is a fault somewhere along the line. To double-check this, you’ll need to double-check your heat controls. Often they’ll get accidentally bumped or adjusted too low, resulting in a false reading.

If this isn’t the issue, it’s important that you call a Gas Safe engineer to take a look. This could be a sign that something has malfunctioned or broken with your boiler systems. That’s not something you want to try and troubleshoot, and it could get worse if ignored.

ST9400C Displays “Internal” or “Internal Fault” Message

This is a sign that something has gone wrong with the internal wiring or components of your thermostat. There isn’t much you can do to fix this, short of calling your installer. There is, however, one fix that the internet has seen fit to gift us with. And it’s a simple one that’ll have you a bit upset you didn’t think of it yourself.

What’s the fix, you ask? Well… have you tried turning it off and back on again?

By that, of course, I mean removing the small battery that’s inside the device. Pulling the battery and cutting the power to the device for ~2 minutes has actually worked for a surprising number of people. You can replace the battery if you’ve got more lying around, but really, who has those just… sitting nearby?

If that doesn’t solve the issue, it’s time to turn to the manufacturer for help.

“Not Valid” Error Message

This error message is a frustrating one, for sure, but it’s got a rather innocuous cause. More often than not, you’ll see a “not valid” error message pop up when trying to adjust the heat. The cause? Either you’re fat-fingering the buttons (I’m very guilty of this), you’re pushing buttons with no direct function in the current mode, or you’re pushing multiple buttons by accident.

To fix this code, do the following:

  1. Verify that the slider is in the proper position and mode for what you’re trying to do.
  2. Ensure no buttons are being pressed that you don’t mean to.
  3. If these aren’t the cause, contact Honeywell for assistance.

Final Thoughts

While error codes are often confusing and frustrating with just about any form of technology, it’s worse when it’s tied to your heating and water. Nobody wants a freezing home in the winter while their thermostat just screams, “NOT VALID” at them over the display. Luckily, there are some easy fixes out there that are, honestly, a bit painful to admit that we miss at times.

Try checking your power supply, performing a hard reset by pulling the battery and power, and ensuring you’re not just accidentally pressing the wrong things. Sometimes the seemingly complicated problems are caused by user error. Other times, though, you’ll need to call your servicer to take a look. If you’ve tried the above steps to no avail, it’s time to give in and get ahold of the manufacturer to ensure something isn’t wrong elsewhere in your home.

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About the Author Harry Thompson

Involved in home renovations throughout his life, Harry is an expert in everything to do with home and garden DIY. In his spare time, he enjoys cooking and tending to his garden.